The present invention relates to a method for decomposing bromic acid, that is, bromate ions contained in a liquid, using a photocatalyst, and an apparatus for the decomposition.
Kurokawa et al. (1986) JNCl, Vol. 77, No. 4, pp. 977-982 describes carcinogenicity of potassium bromate. Bromate ion (BrO3−) can be generated by dissolving potassium bromate in water. Bromate ion can also be produced as a by-product by oxidizing bromide ion (Br−) dissolved in water, in the ozonization or accelerated oxidation treatment of drinking water. Bromate ion is classified by IARC (International Agency for Research on Cancer) as Group 2B of having the possibility of carcinogenicity. In Japan, ozonization has increasingly been used in purification of drinking water in order to eliminate bad smell of drinking water or to reduce the amount of trihalomethane generated as a by-product by disinfection with chlorine. Thus, much attention has been drawn to bromate ion due to its carcinogenicity. The permissible bromate ion concentration of drinking water was set to 25 μg/L by WHO. U.S. Environmental Protection Agency has proposed a permissible bromate ion concentration of 10 μg/L at the first stage of Disinfectant/Disinfection By-product Rule (D/DBPrule) and may propose a stricter concentration at the second stage of D/DBPrule.
Asami et al. (1996) “Mizu Kankyo Gakkai-shi”, Vol. 19, No. 11, pp. 930-936 describes bromate ion formation inhibition by coexisting organic matters in ozonation process. Miyata et al. (1997) “Suido Kyokai Zasshi”, Vol. 66, No. 3, pp. 16-25 describes the removal of bromate ion by particulate activated carbon. Particulate activated carbon, however, may become deteriorated in the removal of bromate ion, as the activated carbon adsorbs thereon dissolved organic matter and the like. The deteriorated activated carbon may require the replacement with new one or reactivation. Furthermore, it has been proposed to suppress the formation of bromate ion by strictly controlling the amount of ozone to be injected into drinking water.
The amount of bromate ion generated by ozonization is known to be substantially in proportion to CT value that is the product of the concentration (C) of dissolved ozone and the ozonization time (T). On the other hand, the degree of disinfection is substantially in proportion to CT value. Thus, CT value is required to be at least a predetermined minimum value in order to have a sufficient disinfection. FIG. 16 shows the change of bromic ion concentration with ozone injection rate by black circles and the change of C*T10 with ozone injection rate by white circles, for destroying Giardia. As shown in FIG. 16, CT value becomes sufficient to destroy Giardia when the ozone injection rate is at least 1.8 mg/L. Under this condition, the bromate ion concentration becomes about 3 μg/L. It may be difficult to avoid the generation of a certain amount of bromate ion in order to sufficiently disinfect drinking water.